The first problem with making a sequel to “Dumb and Dumber” 20 years later — a problem that might have been unavoidable — is that playing an idiot is a young man’s game. Jim Carrey and Jeff Daniels aren’t “old,” exactly (they’re 52 and 59, respectively), but neither do they have the youthful vigor and innocence that made Lloyd and Harry such likable morons in 1994. Coming from men in their 50s, the juvenile buffoonery and slapstick often feels wrong somehow, just as it does when children try too hard to appear grown-up.

Once again directed (and co-written) by Bobby and Peter Farrelly, “Dumb and Dumber To” offers a handful of solid laughs as it reunites us with good ol’ Harry and Lloyd, sending them on another cross-country mission to deliver a package to a woman Lloyd has the hots for. (This time it’s Harry’s daughter, whose existence he has just learned of, and whose mother is played by a self-effacing Kathleen Turner.) Carrey and Daniels’ infectious enthusiasm helps considerably — a lack of commitment is not among the film’s problems — but there’s only so much to be done with a tepid, scattershot, overlong screenplay.

The film starts with an amusing explanation for why nothing has happened in Harry and Lloyd’s lives since we last saw them, then pauses for a trip down memory lane (hey, it’s the blind kid with the dead bird!) before getting to the real, overcomplicated story. In need of a kidney and lacking any other blood relatives, Harry must find the daughter he coincidentally just learned about, a 22-year-old imbecile named Penny (Rachel Melvin) who’s been raised by an eminent scientist (Steve Tom) and his gold-digging second wife, Adele (Laurie Holden). Adele and her bro-tastic lover, Travis (Rob Riggle), are plotting to inherit the scientist’s fortune, so Travis must accompany Lloyd and Harry and keep a watchful eye as they drive to El Paso to give Penny a box that supposedly contains a billion-dollar invention.

The screenplay credits a total of six writers. You’ve heard (perhaps very recently) that “too many cooks spoil the broth”? Yeah. You get the impression everyone wrote their material without seeing what the others were doing. That would explain the setups without payoffs, the characters that disappear when they outlive their usefulness, the general sloppiness of tone. Lloyd and Harry’s pranks have a meanness that wasn’t there before, reducing the guys’ lovability. They’re often more bratty than childlike.

Still, the film has its moments. When a cat farts out bird feathers, I don’t care who you are, that’s funny. (The cat’s name is Butthole. Why? Because it has one.) It’s a movie reviewer’s cliche to say that trimming a comedy by 10 or 15 minutes would vastly improve it. And like most cliches, this one is true. Perhaps unrestrained by anything other than the promise to deliver a PG-13 movie, the Farrellys let themselves wander, undisciplined, through the convoluted story with grins on their faces but no purpose in their eyes. Like another recent comedy sequel, “Anchorman 2,” it’s obvious they threw everything they had into it, regardless of whether it would work. If there’s a “Dumb and Dumber 3,” it’ll need to be more selective than this middling mish-mash.